Two-dimensional Semiconductor Nanostructures Research Articles

Quantum-optical excitations of semiconductor nanostructures in a microcavity using a two-band model and a single-mode quantum field

We theoretically study quantum-optical properties of one- and two-dimensional semiconductor nanostructures, where the electronic band structure is described by a two-band tight-binding model. Since the focus of our work is on analyzing quantum-optical effects of systems with a continuous band structure, many-body processes like electron-electron and electron-phonon interactions are neglected. The quantum-optical interband excitations are fully incorporated and described by a Jaynes-Cummings-type model. The exciting quantum light can be a state with arbitrary photon statistics. For simplicity, the results discussed here are limited to single-photon and two-photon Fock states. Our analytical approach is based on the eigenvalue problem and can be utilized to obtain explicit expressions for the steady-state properties of the system. Numerical simulations are based on solutions of the equations of motion and allow for a more precise analysis of the dynamics and nonresonant excitations of the system. We demonstrate that during the interaction process, a collective excitation of the conduction band is formed. For nonresonant excitations, this collective dynamics results in interesting steady states, in which the resonantly addressed eigenstates are occupied. The presented model provides a framework for microscopic simulations of quantum-optically excited extended semiconductor nanostructures.

Study of structure-property relationship of semiconductor nanomaterials by off-axis electron holography

Abstract As the scaling down of semiconductor devices, it would be necessary to discover the structure-property relationship of semiconductor nanomaterials at nanometer scale. In this review, the quantitative characterization technique off-axis electron holography is introduced in details, followed by its applications in various semiconductor nanomaterials including group IV, compound and two-dimensional semiconductor nanostructures in static states as well as under various stimuli. The advantages and disadvantages of off-axis electron holography in material analysis are discussed, the challenges facing in-situ electron holographic study of semiconductor devices at working conditions are presented, and all the possible influencing factors need to be considered to achieve the final goal of fulfilling quantitative characterization of the structure-property relationship of semiconductor devices at their working conditions.

Acoustically Driven Stark Effect in Transition Metal Dichalcogenide Monolayers.

The Stark effect is one of the most efficient mechanisms to manipulate many-body states in nanostructured systems. In mono- and few-layer transition metal dichalcogenides, it has been successfully induced by optical and electric field means. Here, we tune the optical emission energies and dissociate excitonic states in MoSe2 monolayers employing the 220 MHz in-plane piezoelectric field carried by surface acoustic waves. We transfer the monolayers to high dielectric constant piezoelectric substrates, where the neutral exciton binding energy is reduced, allowing us to efficiently quench (above 90%) and red-shift the excitonic optical emissions. A model for the acoustically induced Stark effect yields neutral exciton and trion in-plane polarizabilities of 530 and 630 × 10-5 meV/(kV/cm)2, respectively, which are considerably larger than those reported for monolayers encapsulated in hexagonal boron nitride. Large in-plane polarizabilities are an attractive ingredient to manipulate and modulate multiexciton interactions in two-dimensional semiconductor nanostructures for optoelectronic applications.

Direct patterning of periodic semiconductor nanostructures using single-pulse nanosecond laser interference.

We demonstrate an effective method for fabricating large area periodic two-dimensional semiconductor nanostructures by means of single-pulse laser interference. Utilizing a pulsed nanosecond laser with a wavelength of 355 nm, precisely ordered square arrays of nanoholes with a periodicity of 300 nm were successfully obtained on UV photoresist and also directly via a resist-free process onto semiconductor wafers. We show improved uniformity using a beam-shaping system consisting of cylindrical lenses with which we can demonstrate highly regular arrays over hundreds of square micrometers. We propose that our novel observation of direct pattern transfer to GaAs is due to local congruent evaporation and subsequent droplet etching of the surface. The results show that single-pulse interference can provide a rapid and highly efficient route for the realization of wide-area periodic nanostructures on semiconductors and potentially on other engineering materials.

Unraveling the Growth Mechanism Forming Stable γ-In2S3 and β-In2S3 Colloidal Nanoplatelets

Inorganic two-dimensional semiconductor nanostructures intrigue the scientific community because of their tunable and sparse electronic states and their high conductivity. The current work deals with colloidal nanoplatelets (NPLs) based on In2S3 compound, focusing on the growth mechanism that leads to the formation of two different phases, trigonal γ-In2S3 and defect spinel β-In2S3, both stabilized at room temperature and characterized by ordered metal voids. In particular, we substantiate the experimental factors (e.g., temperature, reaction duration, and surface ligands) that control the growth progress. The results indicated the formation of hexagonal NPLs of the γ-phase at an elevated temperature and dodecagon NPLs of the γ-phase at a lower temperature. A long-reaction duration time transformed the hexagons/dodecagons into truncated triangular shapes. Furthermore, the analysis of thermodynamic and kinetic factors indicated a phase transformation from the γ-phase to the β-phase. All phases were produce...

Optical Activity of Semiconductor Gammadions beyond Planar Chirality.

We present rigorous analysis of optical activity of chiral semiconductor gammadions whose chirality in three dimensions is caused by the nonuniformity of thickness in the transverse plane. It is shown that such gammadions not only distinguish between the two circular polarizations upon scattering and reflection of light, like all two-dimensional semiconductor nanostructures with planar chirality do, but also exhibit circular dichroism and circularly polarized luminescence. Chiral semiconductor gammadions whose charge carriers are mostly confined to the arms are found to feature both high dissymmetry of optical response and a constant-sign circular dichroism signal over a wide frequency range. It is also shown that the strength of the gammadion's chiroptical response is determined solely by two geometric factors: the variation range of the gammadion's thickness and the arms' curvature. Our seminal theoretical study is intended to lay the foundation for future applications of semiconductor gammadions in chiral nanophotonics and nanotechnology.

Lattice Strain Effect on the Band Offset in Single-Layer MoS2: An Atomic-Bond-Relaxation Approach

Two-dimensional molybdenum disulfide (MoS2) attracts a great deal of interest owing to its potential application in the next generation of electronic devices in recent years. However, the physical mechanism on the strain engineering for the band offset in single-layer MoS2 from the atomistic origin is still a challenge. Herein, we propose an analytical model to address the band offset in single-layer MoS2 modulated by the uniaxial tensile strain based on atomic-bond-relaxation consideration. It was found that the bandgap of single-layer MoS2 shows an approximately linearly red shift with a rate of ∼53.4 meV/% strain under uniaxial tensile strain. The underlying mechanism can be attributed to the variation of crystal potential induced by the changes of bond identities such as bond length, strength, and angle. The results were validated by comparing them with the available evidence, suggesting that the proposed model can be an effective method to clarify the modulation mechanism of relevant electronic properties in two-dimensional semiconductor nanostructures.

Electric-field-controlled effects of spatial recurrence and multiplication of electron waves in two-dimensional semiconductor nanostructures

Effects of recurrence and multiplication in the spatial distribution of the probability-flux density j x(x, z) (or the quantum-mechanical current density ej x(x, z), where e is the elementary charge), which arise from electron-wave interference in two-dimensional semiconductor nanostructures, are analyzed, and the possibility of controlling these effects by the application of a dc transverse electric field is examined. A type of nanostructure represented by two rectangular quantum wells (a wide one and a narrow one) whose widths are measured in the direction of the z axis (the quantum-confinement axis) with the wells arranged sequentially in the direction of propagation of the electron wave (the x axis) is considered. It is shown that, for an electron wave entering the wide well from the narrow well, the initial transverse distribution peak j x(0, z) is reproduced with some accuracy at distances X p = pX 1 (recurrence) and, in nanostructures symmetric along the z axis, splits at distances X 1/q into q identical peaks of magnitude reduced by a factor of q (multiplication) (here, p and q are integers). It is demonstrated that these effects can be controlled by a dc electric field applied in the transverse direction (along the z axis) in the region of the wide quantum well. A reduction in the effective well width and appearance of asymmetry in the transverse potential profile upon application of the electric field cause a radical change in the j x(x, z) distribution in this quantum well and make possible inverse population of the quantum-confinement subbands.

Entanglement dynamics of electron-electron scattering in low-dimensional semiconductor systems

We perform the quantitative evaluation of the entanglement dynamics in scattering events between two indistinguishable electrons interacting via the Coulomb potential in one- and two-dimensional semiconductor nanostructures. We apply a criterion based on the von Neumann entropy and the Schmidt decomposition of the global state vector suitable for systems of identical particles. From the time-dependent numerical solution of the two-particle wave function of the scattering carriers we compute their entanglement evolution for different spin configurations: two electrons with the same spin, with different spin, and singlet and triplet spin states. The procedure allows us to evaluate the mechanisms that govern entanglement creation and their connection with the characteristic physical parameters and initial conditions of the system. The cases in which the evolution of entanglement is similar to the one obtained for distinguishable particles are discussed.

Effects of Spatial Reproduction as a Result of the Interference of Electron Waves in Two-Dimensional Semiconductor Nanostructures with Parabolic Quantum Wells

Effects of spatial nonuniformity for the probability flux density jx(x, z) or for the density of the quantum-mechanical current ejx(x, z), where e is the elementary charge) are studied. These effects arise in two-dimensional semiconductor nanostructures that consist of thin rectangular and wide parabolic quantum wells that are consecutively arranged in the direction of the electron-wave propagation (the x axis) and are oriented along the dimensional-quantization axis z. A nonuniform distribution of jx(x, z) arises as a result of interference of electron waves that propagate simultaneously in a wide quantum well over different quantum-dimensional subbands. Particular attention is paid to the effects of the spatial reproduction of electron waves in the nanostructures under consideration. It is shown that the transverse distribution jx(0, z) existing at the entry to the wide quantum well is reproduced to a certain accuracy at a distance of X1 from the entry. In addition, the initial distribution jx(0, z) is reproduced periodically in the sections Xq = qX1 (coefficients q are integers). The results of numerical calculations of magnitudes of these effects in the structures that are symmetric with respect to the z axis are reported; a modification of the effects under consideration in asymmetric nanostructures is considered.

Effective Frenkel Hamiltonian for optical nonlinearities in semiconductors: Application to magnetoexcitons

Closed Green-function expressions for the third-order response of semiconductors are derived by mapping the two-band model onto the much simpler molecular (Frenkel) Hamiltonian. The signatures of two-exciton resonances are incorporated through the exciton scattering matrix, totally avoiding the explicit calculation of two-exciton states. Exact expressions for the nonlinear optical response of two-dimensional semiconductor nanostructures in a strong perpendicular magnetic field are derived by truncating at the $n=1$ Landau level, and using the symmetry of the system and a group-theoretical analysis. We find that the nonlinear optical response depends crucially on asymmetries between particle-particle and particle-hole Coulomb interactions.

Many-electron effects on ballistic transport.

A Thomas-Fermi-Dirac--von Weizs\"acker density-functional formalism is used to study the effects of many-electron Coulomb interactions on quantum transport through two-dimensional semiconductor nanostructures. The electron density is obtained by direct minimization of the total energy functional, and an effective potential for the electrons is determined as a functional of the density self-consistently. Transmission coefficient and conductance are computed with the effective potential included. The electron density distribution as well as the effective potential are strongly affected by the average electron density and the distance between the two-dimensional electron gas and the positive background charge. The transmission property of a stadium-shaped open quantum-dot system is investigated by varying these system parameters. The electron ballistic transport problem is solved in the presence of the many-electron effective potential and results are compared to that of the single-electron approximation. Some important differences are observed.